Recent years have seen a considerable upsurge in concern for the release of certain undesirable products of combustion into the atmosphere as a result of the operation of internal combustion engines. Among the undesirable products of combustion which are the subject of such concern are oxides of nitrogen as, for example, nitric oxide and nitrogen dioxide, which are referred to generically as "NO.sub.x ".
While there are many ways to measure and express NO.sub.x emissions from internal combustion engines, the most common are as follows:
1. Volume concentration or PPM, that is, Moles NO.sub.x .div.10.sup.6 Moles exhaust;
2. Work Specific or grams NO.sub.x per horsepower hour;
3. Fuel Specific or grams NO.sub.x per gram of fuel burned;
4. Distance Specific or grams NO.sub.x per mile; and
5. Time Specific, grams NO.sub.x per hour.
Typicaly, raw emissions are measured as a volume concentration but to meaningfully assess the impact of NO.sub.x emissions, the same should be related on a specific basis. In particular, to simply know how much NO.sub.x is released does not tell the whole story as such a measurement does not measure the benefits to society as a result of the generation of the NO.sub.x as a by-product of engine usage. It is much more significant to know how much NO.sub.x is released into the atmosphere for a given amount of useful work or a given amount of fuel burned. Thus, Work Specific and Fuel Specific determinations of NO.sub.x emissions provide the most useful and complete information.
Unfortunately, before emissions can be calculated on a specific basis, information in addition to volume concentration, which information is often difficult to measure, must be obtained. To determine a Work Specific emission number, there is required the measurement of exhaust flow and power output. Fuel specific emission determinations require the measurement of exhaust flow and the fuel consumption rate. In laboratories, the obtaining of such numbers is not particularly difficult although sophisticated dynanometer and flow measuring equipment will be required. But the sophistication of equipment required makes in-the-field determinations cumbersome, expensive, and extremely difficult.
In July of 1979, the U.S. Environmental Protection Agency, in the Federal Register of July 23, 1979, proposed a method of determining NO.sub.x emissions on a volume concentration basis corrected to 15% oxygen. In effect, there was proposed a Fuel Specific determination that could be obtained without requiring the measurement of exhaust flow or fuel consumption rate. The technique is based on knowledge of basic combustion, that is, when known quantities of fuel and air are combusted in a lean burning engine, the exhaust product volume concentrations can be easily calculated. A complete derivation of the technique is found in Stationary Internal Combustion Engines--Standard Support and Environmental Impact Statement Volume I; Proposed Standards of Performance EPA--450/2-78-125a July 1979, Appendix C. The technique yields NO.sub.x levels on a Fuel Specific basis corrected to 15% oxygen.
It also illustrates that Fuel Specific NO.sub.x concentrations corrected to 15% oxygen can be obtained by multiplying a measured NO.sub.x volume of concentration obtained in a standard analyzer by 5.9 and dividing the resultant product by 20.9 less the percentage of oxygen measured in the same sample.
However, a certain drawback exists in such a method. At light engine loads, in Diesels, exhaust oxygen concentration may be very nearly equal to 20%. This is quite close to the ambient oxygen concentration of 20.9%. Thus, a small error in the measurement of oxygen in the exhaust can result in a larger percentage error in the calculated NO.sub.x corrected to 15% oxygen.
Some attempts have been made to overcome the foregoing difficulties, particularly in terms of obtaining, in the field, samples suitable for accurate analysis. See, for example, the paper entitled "The Monitoring of Diesel Pollutants in Underground Mines" by David H. Carlson and John H. Johnson, delivered at the annual meeting of the Society of Mining Engineers of AIME in January of 1979. While an improvement, the technique therein described is not altogether satisfactory in that it requires the use of two sampling containers, along with a supply of dry nitrogen and has measurement uncertainties associated with determining dilution ratios of the exhaust gas being measured.
The present invention is directed to overcoming one or more of the above problems.